Figure 3.

Maintained succinate dehydrogenase (SDH) activity in diaphragm muscle fibers. (A) Typical examples of diaphragm cryosections stained for fast-myosin heavy chain (MyHC, top) and for SDH activity (bottom). Blue, fast-MyHC; red, wheat germ agglutinin, indicating cell membranes. The circles represent corresponding slow-twitch (open) and fast-twitch (solid) fibers in control patients, and the squares represent corresponding slow-twitch (open) and fast-twitch (solid) fibers in critically ill patients. (B and C) SDH activity of diaphragm muscle fibers was comparable in both slow- and fast-twitch fibers in critically ill patients compared with control patients. Each data point represents the mean SDH activity of all slow-twitch or fast-twitch fibers measured in cryosections of a diaphragm biopsy of one patient. Data are presented as median and interquartile range. (D and E) The area of the cryosections covered by slow- and fast-twitch fibers (fiber area fraction) and the distribution of slow- and fast-twitch fibers (fiber type fraction) were preserved, suggesting that maintained respiration in critically ill patients was not caused by a fiber type shift toward slow-twitch muscle fibers. Data are presented as mean ± SEM. Crit. ill = critically ill; NS = not significant.